The present invention pertains to preparation of particular rare earth oxyhalide phosphor materials activated with thulium exhibiting superior optical resolution for the light image produced therefrom when used in x-ray image converter devices. Conventional phosphors of this type still produce considerable loss in image resolution when the particle size of the phosphor crystals exceeds approximately 10 microns diameter and such overly coarse phosphor crystals are generally attributable to the method of phosphor preparation. It would be desirable, therefore, to provide an improved method to prepare these phosphor materials which produces still better shaped phosphor crystals of smaller particle size for resulting improvement in this product application.
Recrystallization is an already well recognized method of controlling the particle size of inorganic phosphor crystals. For rare earth oxyhalide phospors, there is disclosed in U.S. Pat. No. 3,591,516 which is assigned to the assignee of the present invention, a method to produce well formed crystals of these oxyhalide materials by recrystallization in molten alkali metal halide fluxes but the particle size of the phosphor crystals is reported to exceed 10 microns diameter. Undesirable particle size increase for these thulium activated phosphors has now been discovered with decreasing activator level. More particularly, in my recently issued U.S. Pat. No. 4,478,933 there is disclosed a class of said rare-earth oxyhalide phosphors activated with thulium ion alone at a concentration in the range from about 0.0005 to 0.001 mole per mole of the phosphor which exhibits reduced crossover effects. Unfortunately, the average particle size range in said phosphor material has been found to exceed 10 microns diameter at the lower end of said activator range and with added difficulties being experienced in maintaining the particle size range at lower values by reducing the recrystallization temperatures. What has been discovered in lowering the recrystallization temperatures when preparing said phosphor material in molten alkali metal halide fluxes is that a greater variation occurs in the recrystallized phosphor particle size due to a variety of such factors as viscosity variation of the molten flux, localized temperature fluctuations, and still other undersirable effects.
In still another recently issued U.S. Pat. No 4,315,979, also assigned to the present assignee, there is disclosed a recrystallization method for these phosphor materials using an alkali metal halide flux mixture whose halides correspond to the selected oxyhalide phosphor and carrying out said recrystallization at temperatures above the eutectic melting temperature of the flux mixture. In an alternate method of said phosphor preparation, the alkali metal carbonates of the selected alkali metal halides are added as starting materials before the initial heating step so that said alkali metal halides are formed in situ during said initial heating step. The phosphor crystals obtained in this manner are said to consist essentially of recrystallized phosphor crystals having an average median particle size range not below about 2 microns and not greater than about 16 microns for less light scattering and light absorption.
It would be desirable, therefore, to still further reduce the particle size and particle size distribution of this phosphor material during the phosphor preparation for improved optical resolution of x-ray image converters using this phosphor. It would also be desirable to do so in the manner not requiring elaborate modification of the phosphor preparation method now commonly used for superior x-ray image converter performance in other respects. It would be still further desirable to alter the shape of the individual phosphor particles during preparation as additional means to provide said improved performance.